Photo by Gracey, posted on morgueFile
To make butter easier to cream with sugar or flour, take the amount of butter required directly from the refrigerator and pop it into the microwave on Medium-Low (30%) power for a few seconds. (it will take about 30 seconds to soften 125 mL (1/2 cup) of butter on the defrost or Medium setting in a 700 watt oven). To soften a 454 gram (1 pound) block, cut it into quarters before microwaving.
Remember these tips to Keeping Butter Fresh